The present invention relates to a method for manufacturing a polymeric inner liner for a storage tank, in particular for a hydrogen storage tank.
Compressed gas is becoming an increasingly popular choice for transportation fuels. As more and more vehicles use compressed gas as fuel, more emphasis needs to be put on the emissions of the fuel systems of such vehicles. Currently, there is a requirement from compressed gas fuel systems that addresses the safety risks of gas permeation from such a system. There is, however, currently no standard that addresses the environmental concerns of compressed gas permeation. When such a standard is introduced, it is likely to be much more restrictive than the current emissions standard.
The vast majority of the compressed gas tanks used on transportation vehicles today is designed to store compressed methane gas (CH4) and is of type III or type IV. There is however, for environmental reasons, a desire to move to compressed hydrogen (H2) as fuel. There are currently no production vehicles being fueled by compressed hydrogen.
Because of the larger molecule size of methane, compared to hydrogen, conventional type II, III and IV tanks meet the current permeation requirements when filled with compressed methane gas. For compressed hydrogen, these conventional tanks do however not necessarily meet the requirements, especially if, as expected, more restrictive requirements are introduced.
Storage tanks have been classified in different categories. Type II tanks concern all steel tanks, which are generally rather heavy and therefore not favored in relation with vehicles. Type III tanks are composite tanks with aluminum liners. These tanks show excellent emission results but are rather expensive. Type IV tanks are composite tanks with polymer liners. Type IV tanks also have very good emission results but, compared to the type III tanks, these tanks have the further advantage of being lighter and less expensive. Therefore, type IV tanks are the most likely candidates for compressed hydrogen storage tanks.
One example of a type IV storage tank is shown in U.S. Pat. No. 5,429,845, which discloses a storage tank comprising a non-metallic inner liner made of plastic or other elastomers and manufactured in one piece by compression molding, blow molding, injection molding or any other generally known technique. The inner liner has a generally cylindrical centre portion and a generally dome-shaped end portion with connections for a metallic communication boss. Another, similar storage tank is disclosed in U.S. Pat. No. 5,476,189, wherein the inner liner has a generally hemispheroidal end section with connections for a metallic communication boss.
It is also know, namely from WO 03/031860, to manufacture a pressure vessel having an outer reinforcing layer and an inner liner comprising hemispherical end caps butt welded on a cylindrical body. This structure has the advantage that metallic connecting parts (namely for fixing a valve allowing the introduction of a fluid under pressure in the container) can easily be over molded by the end caps while the cylindrical body can be obtained by conventional molding techniques. However, such a liner is a monolayer polypropylene one, which leads to bad results in terms of permeability.
It has also been proposed, namely in DE 103 60 953, to use a similar structure (inner liner with outer glass or carbon fiber composite) but with a multilayer liner based on HDPE and EVOH which gives much better results in terms of permeability. However, the method of fabrication of the liner disclosed in that document is rather complicated because the liner is made in one piece by multiple step injection molding.
Combining the teaching of both documents (i.e. choosing a liner having a cylindrical body and welded end caps, but where the cylindrical body is made of a multilayer structure) would provide both an improved method for manufacturing an inner liner for a storage tank and an improved inner liner for getting an improved storage tank.
However, welding of multilayer structures generally lead to connection problems in the welding zone (namely in terms of mechanical performances and permeability).